1. Field of the Invention
The invention relates to an oscillator, and more particularly, to a voltage controlled oscillator.
2. Description of the Prior Art
The performance of voltage controlled oscillator (VCO) can be evaluated with phase jitter, phase noise or KVCO (the proportion of the output oscillating frequency variance corresponding to the input control voltage Vin variance). Generally, if the amplitude of the output signal of the voltage controlled oscillator is larger or similar to square wave, the jitter and phase noise of the output signal will be also smaller; oppositely, if the amplitude is smaller, the jitter and phase noise of the output signal will be larger. Hence, if the amplitude of the output signal can be kept stable while changing the input voltage for controlling the output frequency, the signal to noise ratio (SNR) can be retained. It's an important requirement for the voltage controlled oscillator with wild amplitude range.
Please refer to the text “Design of analog CMOS integrated circuits” written by Prof. Behzad Razavi, in chapter 14, page 513, the FIG. 14.42 describes an oscillator. FIG. 1 shows a differential pair of the delay cell in a ring oscillator, and this technology, which is well known by the person of ordinary skill in the art, changes the control voltage Vcont of the transistors which are used as a loading device to control the oscillating frequency of the ring oscillator. However, the amplitude of the output signal Vout will be changing accordingly, and this is something that a designer of voltage controller oscillators would not be willing to see